Circadian physiology is regulated by a cell-autonomous transcriptional negative feedback loop that influences multiple aspects of behavior. Disturbances in circadian rhythm underlie problems such as industrial and automobile accidents and sleep disorders. A detailed understanding of the function of the clock may aid in the mitigation of these problems. The genes and proteins that make up the machinery of the mammalian circadian rhythms are being identified through genetic approaches. The molecular mechanisms that underlie the intrinsic 24-hour clock can now be analyzed through the study of these regulatory elements in vivo and in vitro. Mutations in casein kinase I epsilon (CKIepsilon) both in flies and hamsters have recently been found to significantly dysregulate clock function, demonstrating that protein phosphorylation by a specific protein kinase plays a critical in the regulation of the clock. Initial mechanistic studies have suggested that CKIepsilon binds to the PERIOD proteins and through phosphorylation regulates their stability and nucleocytoplasmic trafficking. This project will examine specific mechanisms by which phosphorylation regulates the function of the circadian clock by a combination of molecular and genetic approaches.